Download Lec. 1 Physiology of the Kidneys: After studying these lectures, you

Lec. 1
Physiology of the Kidneys:
After studying these lectures, you should be able to . . .
1. Describe the different regions of the nephron tubules and the location
of the tubules in the kidney.
2. Describe the structural and functional relationships between the
nephron tubules and their associated blood vessels.
3. Describe the composition of glomerular ultrafiltrate and explain how
it is produced.
4. Explain how the proximal convoluted tubule reabsorbs salt and water.
5. Describe active transport and osmosis in the loop of Henle and explain
how these processes produce a countercurrent multiplier system.
6. Explain how the vasa recta function in countercurrent exchange.
7. Describe the role of antidiuretic hormone (ADH) in regulating the final
urine volume.
8. Describe the mechanisms of glucose reabsorption and define the
terms transport maximum and renal plasma threshold.
9. Define the term renal plasma clearance and explain why the clearance
of inulin is equal to the glomerular filtration rate.
10. Explain how the clearance of different molecules is determined and
how the processes of reabsorption and secretion affect the clearance
measurement.
11. Describe the mechanism of Na++reabsorption in the distal tubule
and explain why this reabsorption occurs together with the secretion of
K+.
12. Describe the effects of aldosterone on the cortical portion of the
collecting duct and explain how aldosterone secretion is regulated.
13. Explain how activation of the reninangiotensin- aldosterone system
results in the stimulation of aldosterone secretion.
14. Explain how the interaction between plasma K+ and H+
concentrations affects the tubular secretion of these ions.
15. Describe the role of the kidneys in the regulation of acid-base
balance.
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Function of the kidneys
1. Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs,
and Hormone Metabolites.
The kidneys are the primary means for eliminating waste products of
metabolism that are no longer needed by the body. These products
include urea (from the metabolism of amino acids), creatinine (from
muscle creatine), uric acid (from nucleic acids), end products of
hemoglobin breakdown (such as bilirubin), and metabolites of various
hormones. These waste products must be eliminated from the body as
rapidly as they are produced. The kidneys also eliminate most toxins and
other foreign substances that are either produced by the body or
ingested, such as pesticides, drugs, and food additives.
2. Regulation of Water and Electrolyte Balances. For maintenance of
homeostasis, excretion of water and electrolytes must precisely match
intake. If intake exceeds excretion, the amount of that substance in the
body will increase. If intake is less than excretion, the amount of that
substance in the body will decrease.
3. Regulation of Acid-Base Balance. The kidneys contribute to acid-base
regulation, along with the lungs and body fluid buffers, by excreting
acids and by regulating the body fluid buffer stores.
4. Regulation of Erythrocyte Production. The kidneys secrete
erythropoietin, which stimulates the production of red blood cells. One
important stimulus for erythropoietin secretion by the kidneys is
hypoxia.
5. Regulation of 1,25–Dihydroxyvitamin D3 Production. The kidneys
produce the active form of vitamin D, 1,25- dihydroxyvitamin D3, by
hydroxylating this vitamin at the “number 1” position.
6. Glucose Synthesis. The kidneys synthesize glucose from amino acids
and other precursors during prolonged fasting, a process referred to as
gluconeogenesis. The kidneys’ capacity to add glucose to the blood
during prolonged periods of fasting rivals that of the liver.
7. Regulation of Arterial Pressure. The kidneys play a dominant role in
long-term regulation of arterial pressure by excreting variable amounts
of sodium and water. The kidneys also contribute to short-term arterial
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pressure regulation by secreting vasoactive factors or substances, such
as renin, that lead to the formation of vasoactive products.
Physiologic Anatomy of the Kidneys
General Organization of the Kidneys and Urinary Tract
The two kidneys lie on the posterior wall of the abdomen, outside the
peritoneal cavity. Each kidney of the adult human weighs about 150
grams and is about the size of a clenched fist. The medial side of each
kidney contains an indented region called the hilum through which pass
the renal artery and vein, lymphatic, nerve supply, and ureter, which
carries the final urine from the kidney to the bladder, where it is stored
until emptied. The kidney is surrounded by a tough, fibrous capsule that
protects its delicate inner structures.
The nephron is the functional unit of the kidney responsible for the
formation of urine. Each kidney contains more than a million nephrons.
A nephron consists of small tubes, or tubules, and associated small
blood vessels. Fluid formed by capillary filtration enters the tubules and
is subsequently modified by transport processes; the resulting fluid that
leaves the tubules is urine.
Renal Blood Vessels
Arterial blood enters the kidney through the renal artery, which divides
into interlobar arteries that pass between the pyramids through the
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renal columns. Arcuate arteries branch from the interlobar arteries at
the boundary of the cortex and medulla. A number of interlobular
arteries radiate from the arcuate arteries into the cortex and subdivide
into numerous afferent arterioles, which are microscopic.
The afferent arterioles deliver blood into glomeruli—capillary networks
that produce a blood filtrate that enters the urinary tubules. The blood
remaining in a glomerulus leaves through an efferent arteriole, which
delivers the blood into another capillary network—the peritubular
capillaries surrounding the renal tubules.
This arrangement of blood vessels is unique. It is the only one in the
body in which a capillary bed (the glomerulus) is drained by an arteriole
rather than by a venule and delivered to a second capillary bed located
downstream (the peritubular capillaries).
Blood from the peritubular capillaries is drained into veins that parallel
the course of the arteries in the kidney. These veins are called the
interlobular veins, arcuate veins, and interlobar veins. The interlobar
veins descend between the pyramids, converge, and leave the kidney as
a single renal vein, which empties into the inferior vena cava.
Blood flow to the two kidneys is normally about 22 per cent of the
cardiac output, or 1100 ml/min. as we said; the renal artery enters the
kidney through the hilum and then branches progressively to form the
radial arteries and afferent arterioles. The distal ends of the capillaries of
each glomerulus coalesce to form the efferent arteriole, which leads to a
second capillary network, the peritubular capillaries, that surrounds the
renal tubules.
The renal circulation is unique in that it has two capillary beds, the
glomerular and peritubular capillaries, which are arranged in series and
separated by the efferent arterioles, which help regulate the hydrostatic
pressure in both sets of capillaries. High hydrostatic pressure in the
glomerular capillaries (about 60 mm Hg) causes rapid fluid filtration,
whereas a much lower hydrostatic pressure in the peritubular capillaries
(about 13 mm Hg) permits rapid fluid reabsorption.
By adjusting the resistance of the afferent and efferent arterioles, the
kidneys can regulate the hydrostatic pressure in both the glomerular and
the peritubular capillaries, thereby changing the rate of glomerular
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filtration, tubular reabsorption, or both in response to body homeostatic
demands.
Nephron Tubules
The tubular portion of a nephron consists of a glomerular capsule, a
proximal convoluted tubule, a descending limb of the loop of Henle, an
ascending limb of the loop of Henle, and a distal convoluted tubule.
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The glomerular (Bowman’s) capsule surrounds the glomerulus. The
glomerular capsule and its associated glomerulus are located in the
cortex of the kidney and together constitute the renal corpuscle. The
glomerular capsule contains an inner visceral layer of epithelium around
the glomerular capillaries and an outer parietal layer. The space
between these two layers is continuous with the lumen of the tubule
and receives the glomerular filtrate.
Filtrate that enters the glomerular capsule passes into the lumen of the
proximal convoluted tubule. The wall of the proximal convoluted tubule
consists of a single layer of cuboidal cells containing millions of microvilli;
these microvilli increase the surface area for reabsorption. In the process
of reabsorption, salt, water, and other molecules needed by the body
are transported from the lumen, through the tubular cells and into the
surrounding peritubular capillaries.
The glomerulus, glomerular capsule, and convoluted tubule are located
in the renal cortex. Fluid passes from the proximal convoluted tubule to
the loop of Henle. This fluid is carried into the medulla in the
descending limb of the loop and returns to the cortex in the ascending
limb of the loop. Back in the cortex, the tubule again becomes coiled and
is called the distal convoluted tubule. The distal convoluted tubule is
shorter than the proximal tubule and has relatively few microvilli. The
distal convoluted tubule terminates as it empties into a collecting duct.
The two principal types of nephrons are classified according to their
position in the kidney and the lengths of their loops of Henle. Nephrons
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that originate in the inner one-third of the cortex—called juxtamedullary
nephrons because they are next to the medulla—have longer loops of
Henle than the more numerous cortical nephrons, which originate in the
outer two thirds of the cortex. The juxtamedullary nephrons play an
important role in the ability of the kidney to produce concentrated
urine.
A collecting duct receives fluid from the distal convoluted tubules of
several nephrons. Fluid is then drained by the collecting duct from the
cortex to the medulla as the collecting duct passes through a renal
pyramid. This fluid, now called urine, passes into a minor calyx. Urine is
then funnelled through the renal pelvis and out of the kidney in the
ureter.
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